1. Field of the Invention
The invention described herein pertains to the detection of potentials existing on the surface of the skin of the living body, which potentials are generated by various sources such as muscle or nervous system activity within the body.
2. Discussion of the Background
Present biopotential detection techniques typically involve the use of conductive pastes or gels in combination with a metallic contact surface to form an electrode capable of transforming electrical charge conduction by means of ion diffusion in the body into electrical charge conduction by means of electron motion in the metallic wires of the monitoring apparatus. There are several realizations of this basic type of electrode, and all of them suffer from the various disadvantages of wet systems, such as skin irritation, loss of electrical contact due to drying paste or lead wires falling off, poor shelf life, etc. Many electrodes for use in electrocardiography are designed to withstand high voltage overloads, such as encountered during a defibrillation procedure, by rapid recovery to the original electrode potential. Such performance is required in operating rooms or critical care areas where it is important to maintain constant monitoring of the activity of the heart, and electrodes exhibiting this quality are usually called non-polarizing or reversable. Virtually all of the reversable electrodes currently available utilize an electrolyte gel that is subject to drying, thus suffering from the limitations of wet systems as listed above. Various materials have been introduced in attempts to realize dry electrodes, but none of them have been successful in providing a high performance, non-polarizable dry electrode.
For example, most of the metals and conductive composite materials introduced in the prior art as dry electrodes, such as those disclosed in U.S. Pat. Nos. 3,566,860 and 3,606,881, generate excessive low frequency electrical noise voltages when in contact with a saline solution such as human sweat or blood. Furthermore, many of these materials are too stiff to conform to irregularities in skin surface, thus resulting in an uncomfortable and electrically unstable electrode. Another effort to obtain dry electrodes includes conventional non-conductive pressure sensitive adhesives loaded with fine conductive particles such as carbon power (U.S. Pat. No. 3,911,906) or metal-coated plastic microspheres (U.S. Pat. No. 3,566,059). However, such electrodes suffer from low electrical conductivity to skin resulting in poor signal quality. Furthermore, a method for making such adhesives reversable has not been demonstrated.
In my U.S. Pat. Nos. 4,751,471 and 4,763,659, a dry electrode that is stable in the presence of saline, conformable to body contours, and capable of delivering a high quality signal to any suitable measuring apparatus despite wide ranges in electrode impedance to skin is disclosed. Although this electrode is acceptable for procedures such as diagnostic electrocardiography, a means for making the electrode non-polarizable in the presence of saline body fluids has not been disclosed.
Materials having both adhesive and electrically conductive properties have been introduced for use as dry electrodes, such as disclosed in U.S. Pat. No. 4,273,135. Although it is stated that such materials are suitable for use in defibrillation procedures, they still suffer from limitations in adhesion and require an extra adhesive patch to hold the electrode in place. Again, there is no provision for protecting the electrode from polarization when subject to defibrillation overloads in the presence of saline body fluids. The materials specified for use as connectors all form polarizable electrode interfaces when in contact with ionically conductive solutions such as saline, which could flood through or around the thin (25 to 100 microns) layers of conductive film stipulated for this electrode. A similar problem exists with the electrode construction described in U.S. Pat. No. 4,458,696 and G.B. Patent 2,045,088. The carbon-loaded polymeric connectors as specified form polarizable electrode interfaces when in contact with saline solutions, but the thin layers of adhesive of this electrode do not provide an adequate fluid barrier. An electrode polarization problem also exists with the construction disclosed in U.S. Pat. No. 4,125,110, which shows a thick layer of karaya based adhesive loaded with sodium chloride to obtain ionic conductivity. The electrically conductive backing is obtained from wire mesh, conductive cloth or conductive polymer materials, any of which are polarizable when in contact with dissolved sodium chloride.
Kater (U.S. Pat. No. 3,993,049) discloses a method for realizing a non-polarizable electrode by loading a metal salt into an adhesive and making a backing connector of the metal of the metal salt in the adhesive. Various complicated structures are described, including the use of metal screens and connectors incorporated into the electrode, all of which are difficult to manufacture and not extensible with skin. In addition, the presence of metal salts in material touching the skin increases the risk of skin irritation, and the preferred mode of including metal salts and powders in an adhesive not only increases the risk of skin irritation but further reduces the tack of the adhesive. Other non-polarizable electrodes disclosed in the prior art involve the use of gels with a high water content, as in U.S. Pat. No. 4,235,241, which discloses a non-polarizing backing plate consisting mainly of titanium hydride and silver chloride. Electrical contact to the skin is achieved through an electroconductive cream impregnated with sodium chloride, which is subject to drying out while the electrode may still be in use. The most common reversible electrode is based on a silver connector having a layer of silver chloride deposited on its surface, combined with an electrolyte gel of high water content containing dissolved chloride ions. This is known as the silver/silver chloride electrode, and it has been disclosed in various patents including U.S. Pat. No. 4,377,170. As with Kater above, the basic principle involved in realizing a non-polarizable electrode interface is to combine certain metals and their respective salts within the electrode to enable the exchange of electrons and ions to proceed freely at the interface between the electrode and electrolyte solution, thus allowing large rapid potential changes across the electrode interface without residual polarization. However, no method for realizing such a non-polarizable electrode that is dry, body conformable, adhesive and non-irritating to skin, in the presence of substantial amounts of body fluids such as blood or saline, has heretofore been disclosed.